1. Field of the Invention
This invention relates to a parabolic antenna dish, and more particularly to a parabolic antenna dish having air holes suitable for reducing wind pressure applied to the parabolic surface and reducing the weight of the antenna.
2. Description of the Prior Art
A type of parabolic antenna dish with a radio wave reflecting surface having air holes in the form of a lattice has been known in which a metal plate with punched holes or an expanded metal sheet is used to form the antenna dish's radio wave reflecting surface, with the outer peripheral flange portion thereof being integrally formed of the same member, and in which the antenna surface member is fixed at the flange portion to a separate outer peripheral reinforcement member by means of welding, screws or the like. Another known example of this type of parabolic antenna dish is formed in a manner such that a mesh metal plate having or intended to have a curved surface as a radio wave reflecting surface is attached to an outer peripheral reinforcement member and to radial ribs facing the reverse surface of the radio reflecting surface.
Japanese Patent Laid-Open No. 70740/1978 discloses a parabolic antenna dish in which rows of air holes are disclosed to form coaxial circles, and the area of the air holes relative to the total area of the dish is comparatively small.
Another type of dish for parabolic antennas which has air holes formed by a metal net has been known as a means that is effective for reducing the air pressure. An example of parabolic antennas having this type of air hole is disclosed in Japanese Patent Laid-open No. 173904/1985.
If a welded metal net is used to form the radio wave reflecting surface of this type of dish of parabolic antennas, there is a difference in level 60 in the radio wave reflecting surface, which corresponds to 60 to 90% of the diameter of longitudinal wire rods 42 and lateral wire rods 41, that is, a value obtained by subtracting the bite depth at the welded portion from the wire diameter, as shown in a partial perspective view of FIG. 29 and in a partial cross-sectional view of FIG. 30 taken along a line XXX--XXX of FIG. 29. In the case of a plain weave metal net, there is necessarily a difference in level 61 which generally corresponds to the diameter of longitudinal wire rods 62 and lateral wire rods 63 and which is formed at the peaks and valleys of these combined wire rods in the form of waves, as shown in the partial perspective view of FIG. 31 and in the partial cross-sectional view of FIG. 32 taken along the line XXXII--XXXII in FIG. 31. Such degrees of difference cause a reduction in the antenna gain, and the influence of the same becomes greater as the reception frequency is increased. It is therefore necessary to reduce the diameter of the wire rods and, hence, the difference in level in accordance with the increase in the reception frequency.
As described above, the conventional methods require the provision of separated members such as an outer peripheral reinforcement member, ribs and so forth at the outer peripheral portion of the dish, and they fail to take the consideration of the desirability of a reduction in the weight of the dish or of the need to assure the strength of the dish without attaching any other member thereto. This results in the antenna being difficult to install due to the increased weight of the dish and in the components parts being more costly because of the need for a high degree of accuracy in the manufacture of the other outer peripheral reinforcement member. Furthermore, the distance between the center of the radio wave reflector and the position of each row of air holes disposed in the surface of the reflector is within a range of 30 to 80% of the radius thereof, and the reduction ratio of wind pressure is at most about 32% when calculated on the basis of the method of calculating wind pressure applied to buildings. This effect is not sufficient, and the design with respect to air holes has been heretofore made without any regard for a reduction in the weight of the antenna.